In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) is used. Specifically, the PS plate is exposed through a mask, for example, a lith film, and then subjected to development processing, for example, with an alkaline developer to remove the unnecessary image-recording layer corresponding to the non-image area by dissolving while leaving the image-recording layer corresponding to the image area, thereby obtaining the lithographic printing plate.
Due to the recent progress in the technical field, nowadays the lithographic printing plate can be obtained by a CTP (computer-to-plate) technology. Specifically, a lithographic printing plate precursor is directly subjected to scanning exposure using laser or laser diode without using a lith film and developed to obtain a lithographic printing plate.
With the progress described above, the issue on the lithographic printing plate precursor has transferred to improvements, for example, in image-forming property corresponding to the CTP technology, printing property or physical property. Also, with the increasing concern about global environment, as another issue on the lithographic printing plate precursor, an environmental problem on waste liquid discharged accompanying the wet treatment, for example, development processing comes to the front.
As the development method, a method of developing with a highly alkaline developer is known. However, in response to the environmental problem, simplification of development or plate making or non-processing has been pursued. As one method of simple plate making, a method referred to as an “on-press development” is practiced. Specifically, according to the method after exposure of a lithographic printing plate precursor, the exposed lithographic printing plate precursor is mounted as it is on a printing machine without conducting conventional development and removal of the unnecessary area of image-recording layer is performed at an early stage of printing step.
Also, as a method of simple development, a method referred to as a “gum development” is practiced wherein the removal of the unnecessary area of image-recording layer is performed using not a conventional highly alkaline developer but a finisher or gum solution of near-neutral pH.
In the simplification of plate making operation as described above, a system using a lithographic printing plate precursor capable of being handled in a bright room or under a yellow lump and a light source is preferred from the standpoint of workability. Thus, as the light source, a semiconductor laser emitting an infrared ray having a wavelength of 760 to 1,200 or a solid laser, for example, YAG laser, is used. An UV laser can also be used.
Of the lithographic printing plate precursors, a lithographic printing plate precursor using a radical polymerizable image-recording layer is known. In the lithographic printing plate precursor using a radical polymerizable image-recording layer, an overcoat layer or protective layer which is provided with an oxygen blocking property for preventing inhibition of radical polymerization due to oxygen and a protective function for preventing the occurrence of scratches during transportation or handling is used.
As the lithographic printing plate precursor capable of undergoing on-press development, for instance, a lithographic printing plate precursor having provided on a hydrophilic support, an image-recording layer (heat-sensitive layer) containing microcapsules having a polymerizable compound encapsulated therein and an overcoat layer containing a hydrophilic resin, for example, polyvinyl alcohol is described in JP-A-2001-277740 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-2001-277742. A lithographic printing plate precursor capable of undergoing on-press development having provided on a support, an image-recording layer containing a polymerizable compound and a graft polymer having a polyethylene oxide chain in its side chain or a block polymer having a polyethylene oxide block and a protective layer containing polyvinyl alcohol is described in U.S. Patent Publication No. 2003/0064318. Further, a lithographic printing plate precursor having provided on a hydrophilic support, an image-recording layer (heat-sensitive layer) containing microcapsules having a polymerizable compound encapsulated therein and an overcoat layer containing a hydrophilic resin, for example, polyvinyl alcohol and an inorganic stratiform compound is described in JP-A-2005-119273. However, the presence of the overcoat layer or protective layer causes decrease in on-press development speed. Also, the hydrophilic resin may deposit on a printing machine in some cases depending on the kind of hydrophilic resin.
On the other hand, as for the lithographic printing plate precursor capable of undergoing development with a developer, for example, a protective layer containing polyvinyl alcohol and a cation-modified polyvinyl alcohol is described in JP-A-2007-298645. A protective layer containing a modified polyvinyl alcohol having an acid group or a salt thereof in its molecule is described in JP-A-2006-53316 and JP-A-2006-259137. Polyvinyl alcohol is ordinarily used in the protective layer, however, when polyvinyl alcohol having high saponification degree is used in order to improve the oxygen blocking property, the occurrence of deposit (development scum) in the developer becomes marked.